We describe the effects of uric acid, hemolysis, drugs, ascorbic acid, lipemia, and bilirubin on the enzymic measurement of cholesterol in serum by use of reagent kits from Abbott, Beckman, Boehringer Mannheim, Calbiochem, and Worthington. In all of these, the chromogen formed from the reaction of hydrogen peroxide with phenol and 4-aminoantipyrene is measured. The absorbance was measured at 500 nm vs. a serum blank for each kit--except Abbott's with which the recorded absorbances were the differences between readings at 500 and 600 nm. With all reagents kits, there was no interference from uric acid up to 200 mg/liter, hemoglobin up to 1.0 g/liter, or drugs (clofibrate, phenobarbital, Ketochol, Ovral-28), but negative interferences from ascorbic acid. Except for the Abbott kit, the cholesterol values obtained for lipemic samples were lower than found with the comparison method [Abell et al., Stand. Methods Clin. Chem. 2, 26 (1958)]. With Abbott's reagents, for most lipemic samples, the values were the same. Bilirubin at concentrations of 200 mg/liter significantly decreased the cholesterol values with Beckman, Calbiochem, and Worthington reagent kits. With Boehringer Mannheim reagent a small negative interference was observed and with Abbott reagent a small positive interference was observed when the bilirubin concentrations were 200 mg/liter.